Foundry cores and molds used in making metal castings are normally prepared from a composition including sand or similar material and a curable or polymerizable binder coated on the sand particles. The purpose of this binder coating is to permit the mixture to be hardened after it is first shaped or molded into a desired form. Typically, after the aggregate material and binder have been mixed, the resulting mixture is rammed, blown or otherwise formed to the desired shape or pattern and then cured with the use of catalysts and/or heat to a solid, cured state.
When molten metal is poured into a sand mold, it solidifies taking the shape or pattern of the mold. The temperature of the molten metal is so high that the resin binder burns out of the mold. The mold then collapses leaving free-flowing sand that can be reused to make a new mold.
Different processes for forming molds and cores have been developed in the foundry industry. One process known as the "baking" process requires that the mixture of aggregate material and binder be shaped in a pattern box and then cured by heat. Another process using a binder system which does not require heating in order to bring about curing or hardening is known as a "no-bake" process. The present invention is directed to an improved resin for use in both processes.
Phenol-formaldehyde resole resins have been used as binders in these processes for making foundry cores and molds. Although the resole resins offer a favorable raw material cost when compared with some other resins used in the processes, they have certain limitations. Resole resins normally have a limited shelf-life and must be used promptly or stored under refrigeration during warm weather. In addition, when the previously-used resole resins are mixed with sand and catalysts, the mixture has to be placed in the mold immediately or it becomes useless.
Phenolic resole resins are also used as refractory binders. A refractory is a ceramic material of low thermal conductivity characterized by the ability to withstand extremely high temperatures. Refractories are used for lining steel furnaces and other high-temperature applications. They are normally cast in the form of brick with some type of binder to assure their stability.
However, conventional resole resins have certain drawbacks when used as refractory binders. This is particularly true when the refractory is prepared from basic material such as magnesia. When magnesia granules are mixed with the resole resins, the mixtures tend to harden in a comparatively short time. This reduces the time that the mix can be retained before it must be shaped into bricks or other desired shapes. Furthermore, the articles produced from these mixtures often show poor strength and durability.
A modified phenolic resole resin has been discovered which does not have the shortcomings of conventional resole resins. This resin can be stored at room temperature for long periods of time without undergoing deterioration. It is useful as a sand binder in both the "baking" and "no-bake" processes for making foundry cores and molds. In addition, it is a good binder for refractories, and refractory mixes of magnesia particles with this resin show excellent storage stability.